In the continuous slip bypass friction clutches implemented to enhance the operating efficiency of hydrokinetic torque converters, one of the clutch interfaces is normally an annular friction reaction surface and the other is an annular, engineered, paper-based friction facing providing predetermined friction characteristics. The clutch is subjected to a continuously slipping operational mode and without adequate cooling by the operating hydraulic fluid in the converter can generate excessively elevated temperatures leading to catastrophic degradation of both the facing material and the operating hydraulic fluid, the latter being a special oil formulation commonly known as ATF (automatic transmission fluid).
To maintain design intent functional characteristics with time subjected to the above described operational node, two critical phenomenon must be satisfied for best lipping clutch performance. One is the ability to efficiently conduct heat away from the interface zone. The other is the ability to maintain a wetted interface zone avoiding potential areas within the zone of so-called "dry friction" that can produce erratic friction characteristics and excessive or uneven wear and result in a significantly shortened clutch life. Moreover, a hydrokinetic torque converter presents specific restrictions on function that most other forms of wet clutches do not experience. For example, space limitation in the converter usually dictates a very limited number of interface zones of relatively large annular area with the most common typically having only one interface zone and a more advanced compact heat resistant design having three (3) such interface zones as disclosed in my U.S. Pat. No. 5,337,867. In such converters with a continuous slip bypass clutch, the hydraulic circuit within the converter typically includes cavities that surround the outer diameter of the interface zone with high pressure hydraulic fluid and the inner diameter thereof with low or zero pressure hydraulic fluid with this condition existing in concert with total assembly rotational velocity. The differential pressure between these cavities is modulated and utilized to apply the bypass clutch to control the slip speed in the clutch while fluid is circulated through the interface clutch zone(s) from the high pressure cavity to the low pressure cavity to wet and cool the clutch interfaces. However, the volumetric flow of oil through the interface zone(s) is typically restricted to very small values (e.g. not exceeding 1 gpm) because of the restrictions imposed upon the hydraulic supply circuit serving the converter circuit and the slipping bypass clutch limiting the ability to both adequately wet and cool the clutch interfaces. Moreover, it is desired that the bypass clutch is capable of extended operational periods in a low velocity slip mode (e.g. 30-100 rpm relative speed) at high interface energy levels (Watts/mm.sup.2) that result in extremely high heat generation in the clutch interface zone(s).
It has been the practice by some to provide a grooveless facing material with a certain porosity for absorbing the hydraulic fluid however this is not a satisfactory solution for avoiding dry friction and is fundamentally lacking in providing for efficient conduction of heat away from the interface zone as there is virtually no oil flow through the interface zone during clutch slip mode. In an attempt to meet both objectives, various forms of groove or channel patterns in the clutch facing material have been proposed such as radial grooves, cross hatch grooves and a combination of annular and radial grooves. Examples of such prior art groove patterns are shown in FIGS. 1-6 of the accompanying drawings. Referring first to FIGS. 1 and 2, there is shown a slipping bypass clutch piston 10 which also serves as a clutch plate and for this purpose has an annular, paper-based facing 12 bonded thereto. The facing 12 is provided with a pattern of radial grooves 14 that open at the outer edge and inner edge of the facing to the aforementioned high pressure cavity and low pressure cavity respectively in the converter to convey hydraulic fluid from the high pressure cavity through the interface zone to the low pressure cavity. Because of flow restrictions, the quantity of radial grooves permissible is limited and, as a result, the ability to effectively influence the control of interface area relative to heat transfer and maintain an adequately wetted interface zone is limited.
Referring next to FIGS. 3 and 4, there is shown a similar bypass clutch piston/clutch plate 16 having a facing 18 provided with a cross hatch pattern of intersecting radial grooves 20 and parallel grooves 22 wherein some of the latter intersect at their opposite ends with the outer diameter of the facing and some extend completely across the facing like the radial grooves but at different angles. And again because of flow restrictions, the quantity of radial grooves and crossing grooves is limited thus limiting the ability to effectively influence the interface area relative to heat transfer and maintain an adequately wetted interface zone. Furthermore, the hydraulic fluid flow in the control circuit for the bypass clutch is normally limited to a low flow rate as mentioned above, and a cross hatch pattern because of its many grooves that traverse the interface zone, lacks the ability to keep the flow through the interface zone under this rate and thus dictates an increased flow for operation of the clutch which is undesirable.
Referring next to FIGS. 5 and 6, there is shown a similar bypass clutch piston/clutch plate 24 having a facing 26 provided with intersecting radial grooves 28 and annular grooves 30 wherein certain of the radial grooves extend from the annular grooves to the outer edge of the facing and certain of the radial grooves extend from the innermost annular groove to the inner edge of the facing. This kind of groove pattern does provide the ability to control flow but the fluid distribution within the interface zone is confined primarily to the annular grooves. And since interface slipping motion is circular, relative large interface areas still risk the potential of dry friction and localized zones of excessive interface temperature. The problem with this type of groove arrangement is in the minimal traversing of interface area in multiple locations in an attempt to maximize the maintenance of wetted interface area throughout the clutch interface zone.